This invention relates to an epoxy resin based adhesive.
Epoxy resin based adhesives are used to bond a variety of different substrates together. In the automotive industry, epoxy resin adhesives are used to bond certain parts together. Structural adhesives are increasingly being used for metal-metal bonding in frame and other structures in automobiles. Adhesive bonding can reduce the number of welds that are needed to construct the frame, and for that reason the use of these adhesives can reduce assembly costs.
Structural adhesives for these applications are subject to very stringent performance requirements. They need to adhere well to metals that are commonly used in automotive frame structures, most commonly cold rolled steel (CRS), various types of galvanized metals, and in some cases aluminum. Any of these substrate materials may be coated with one or more coating materials, including a cationically deposited primer material which is commonly known as an “E-coat”. The adhesive should be resistant to impact forces that can be experienced in collisions. Adhesives of this type are sometimes referred to as “crash durable adhesives”, or “CDAs”. In addition, the adhesive must maintain good bonding to the substrate and good impact resistance over a temperature range that can be as low as −40° C. and up to 80° C.
Impact properties are imparted to the adhesive composition through the use of various rubbers and tougheners. The rubber is typically a liquid polymer or copolymer of a diene monomer, such as a butadiene-acrylonitrile copolymer of the type described in U.S. Pat. No. 6,586,089 and U.S. Published Patent Application No. 2005/0070634. A commonly used toughener is a capped polyurethane and/or polyurea as described in U.S. Pat. No. 5,278,257.
In the automotive assembly process, the frame is typically bonded and welded together, and then subjected to one or more coating steps. Automotive coatings are often baked in order to cure them to produce a hard finish. The baking temperatures can range from 140° C. to over 200° C. It has been found that exposure to these temperatures can cause a deterioration of impact properties and adhesion, particularly if the exposure time to these temperatures is long. This tends to be more apparent at low temperatures. It would be desirable to provide a structural adhesive which bonds well to various metals (including cold rolled steel and various galvanized metal substrates) over a wide temperature range, and which is more resistant to exposure to high temperatures.
Another desired characteristic of these structural adhesives is the mode of failure. Cohesive failure (i.e., failure within adhesive layer rather than at the adhesive-substrate interface or by delamination of the substrate) is the preferred failure mode. In many cases, measures that impart the desired failure mode tend to reduce impact resistance. It would be desirable to provide an adhesive which mainly fails cohesively when bonded to metals such as CRS, galvanized metals and aluminum, and which has good impact properties, particularly at low temperature.